The Syrian regime has “dozens of 500-lb bombs” being armed with Sarin nerve
gas, according to reports of satellite imagery from seen by the Pentagon at
the end of last year.

The bombs were being loaded on to vehicles near Syrian airbases and could be airborne within two hours, if President Bashar al-Assad ordered their use, the officials told President Barack Obama.

The details of the briefing were released to the New York Timesand explain a sudden warning issued by Mr Obama to the Syrian regime at the beginning of December.

His statement, which said that any use of chemical weapons would have “consequences” and that “the world would act”, was followed by private messages to the Syrian authorities from Russia and neighbours including Turkey and Iraq that they would be held “personally responsible”.

Those messages, which diplomats told the paper were coordinated from American, European and Arab capitals, seem to have been enough to deter the weapons’ use for the time being, if it were intended.

Leon Panetta, the US Defence Secretary, who said the regime would cross a “red line” triggering military intervention if it used the bombs, followed up a week later by saying the worst fears were over.

Whether Syria might use chemical weapons has become one of the most contentious issues surrounding the crisis.

Its spokesman, Jihad Makdissi, said in July that the regime would not use such weapons “unless Syria is exposed to external aggression” — a significant formulation given that the authorities repeatedly say the rebellion is funded by “foreign enemies”.

Mr Makdissi himself muddied the waters by disappearing from sight – and allegedly defecting to the United States – shortly before Mr Obama made his threat.

The regime and other critics of western policy claim that chemical weapons reports are a fiction, comparing them to those made for Saddam Hussein’s “weapons of mass destruction” in Iraq a decade ago.

But Rami Jarrah, a Cairo-based Syrian activist, said sources inside the Syrian government had told colleagues the regime was now prepared to use weapons in areas of Aleppo and Idlib from which civilians had fled.

Meanwhile, the humanitarian crisis affecting Syria’s millions of refugees worsened with extreme cold hitting camps across the region. An unauthorised electricity supply being used to power heaters in tents at a camp in Turkey’s Sanliurfa province sparked a fire that killed two children.

A riot broke out in the giant Zaatari camp in the desert of northern Jordan, home to 50,000 refugees, mostly children, after floods and storms washed away tents. The United Nations says there are now 600,000 refugees outside the country, along with many more displaced inside Syria.

The World Food Programme also said it would only be able to reach 1.5 million of the 2.5 million people it intended to supply with food this month, leaving another million hungry.

A worldwide appeal to raise $5 million in private donations to send wheat for making bread was launched by 152 Islamic charities around the world on Tuesday. Activists said shortages of basic necessities were becoming increasingly acute.

“People are in a desperate scramble for food. They can queue for hours for bread, which is 10 times more expensive than it was and come away with just five flat slices which is nothing for large Syrian families,” said Mohammad Najjar, a London-based doctor who carried out a private aid mission to Aleppo at New Year.